In closed pipe systems, for example heating systems, the demand of the loads connected to the pipe system determines the variable throughflow resistance of the entire system. The loads in a heating system are its heating bodies, where the valves of these heating bodies adapt to the particular heat demand and cause a greater or lesser throughflow resistance.
Since the circulation systems addressed here must be designed for maximum transport flow, but often operate only under partial load, the relationship between the required transport pressure and the transport flow of the centrifugal pump has a characteristic which falls in the direction of zero transport flow. However, the centrifugal pumps which up to now have been used in such circulatory systems have a characteristic which rises in the direction of zero transport flow. That is, the pump head increases as the volumetric flow rate decreases.
If one wished to operate such a centrifugal pump in the system with a steady rpm, the difference between the pressure created by the pump and the specific required pressure had to be throttled out, resulting in a substantial energy loss.
Various regulation systems have already been developed for heating systems to obtain a convenient and economical adaptation to demand. The so-called bypass control is known as a mechanical solution. In a bypass control, a portion of the transport flow is withdrawn by a feedback line of the pressure side of the pump and is again returned to the suction side. This measure does indeed reduce the useful transport flow of the system, but the system characteristic does not shift towards larger transport flows.
At the present time, regulation systems have established themselves which control the rpm of the centrifugal pump so as thereby to achieve broad adaption to various control variables, such as the difference pressure, and the external, inflow, or backflow temperature. This regulation is accomplished either by switching to various prescribed rpms, or, with the correspondingly higher complication continuously varying the rpms. What is true about this rpm regulation in general is that it has small losses compared to the devices described above for regulating the transport flow. However, such a regulatory system is not entirely without losses. Incidentally, with such a regulation too, parts of the pressure difference also must be throttled out.